Imagine an object, tossed in the air. When it reaches its peak, before coming down, it appears for an instant to stand still.
Now imagine the sun, rising in the sky over the course of the year, reaching its peak around June 22 (in the Northern hemisphere).
That day is the summer solstice, the longest day of the year. It gets its name from the ancient observation that the sun like the tossed object has reached the peak of its arc.
Solstice comes from a Latin word that means "sun standing still."